The present invention relates to a device for ionizing a material by heating it to an elevated temperature. Such an ionizing device is more particularly used in mass spectrometers used for measuring the isotopic abundance ratios in a given chemical element. These spectrometers are used in the nuclear field, as well as in geochronology, when it is wished to date samples.
The principle of ionization by heating, or thermoionization, consists of depositing atoms of a chemical element on a very hot metal surface in such a way that they are reevaporated in the form of ions, whilst losing an electron. This phenomenon is generally expressed by the formula: ##EQU1## n.degree.: number of neutral atoms evaporated from the hot metal surface, n.sup.+ : number of atoms which, under the same conditions, are reevaporated in the form of monocharged ions, which surrender an electron to the metal,
K: proportionality constant, PA1 e: electron charge, PA1 W: extraction potential or work function of heated metal (represents the capacity of the metal to trap electrons), PA1 .phi.: first ionization potential of the ionized element (capacity of said element to lose a first electron), PA1 k: Boltzmann constant, PA1 T: temperature of the metal surface.
This formula shows that the ionization efficiency n.sup.+ /n.degree. is an exponential function of (W-.phi.)/T. The ionization potential .phi. and the work function W are respectively representative of the material to be ionized and the heated material, so that the influence of the temperature T is of a determinative nature. In particular, when the difference W-.phi. is negative, it is possible to increase the efficiency by increasing the heating of the hot metal surface.
In order to increase the efficiency, the hot metal surface must be made from a material having both a high work function W and a very high melting point. Thus, a refractory metal is chosen from among the group including rhenium, tungsten and tantalum.
The work function W of these metals is between 4.2 and 5.1 V, so that it is necessary to heat the metal surface to the maximum, when the material to be ionized has an ionization potential above said values.
In the present state of the art, solid material ionizing devices utilizing the thermoionization principle generally consist of three tungsten or rhenium tapes or filaments, carried by conductive rods, traversing an insulating support plate. The material to be ionized is deposited on one of these tapes which, with a facing tape, serves to vaporize the material. The third tape, which forms a U with the first two tapes, ensures the actual ionization. To this end, the three tapes are heated by the Joule effect to a maximum teperature of 2500.degree. C.
In the case of such a device, it is very difficult to deposit the material, bearing in mind the very small dimensions and particularly with a view to preventing part of the material being deposited on the other tapes.
Moreover, the tapes must be manually welded to the rods supporting them, which leads to mediocre reproducibility and a high cost price. In addition, the geometrical arrangement of these filaments is such that a small part of the atoms formed strike the intermediate filament ensuring the ionization, so that the ionization efficiency of this device is not very good.
Finally, the use of heating by the Joule effect imposes the presence of support rods constituting heat leaks, which cool the filaments and also lead to a limitation in the cross-section of the latter. In view of this limitation, the filaments break at beyond 2500.degree. C. In addition the temperature limitation also leads to a limitation in the ionization efficiency of the known devices.
It is also pointed out that different types of ionizing devices are used when required for ionizing a solid material (devices described above) and when used for ionizing a gaseous material. In other words, no known device makes it possible to simultaneously ionize a solid material and a gaseous material.